The visible light communication devices (VLC or LiFi, acronyms respectively standing for “visible light communication” and “light fidelity”) use the visible light to transmit information between two distant points. The visible light communication systems generally comprise one or more light-emitting diodes (LEDs) forming an emission means and a photodetector forming a reception means. The LED supplies a light signal in the visible whose intensity is modulated as a function of the data to be transmitted. LED luminaries offer the advantage of allowing the dual function of lighting and of data transmission. Their physical characteristics make it possible to consider data transmission throughputs with throughputs of the order of a gigabit per second (Gbit/s).
Such a communication system is advantageous in that it is possible to use as reception means most of the photodetectors associated with an information processing system making it possible to analyze the variation of the amplitude of the light signal received and deduce therefrom the signal transmitted.
There are a number of types of modulations used to transmit, from LEDs, data that can be received by a photodetector, for example:                a modulation of the light intensity with non-zero mean, the modulation used can then be of NRZ (acronym for “non-return-to-zero”) type. This is a two-state coding, the signal is in one state (for example in the high state) when logic 1s are transmitted, and in the other state (in the low state for example) when logic 0s are transmitted. The photodetector then transcribes the intensity of the light signal received into an electrical signal corresponding to the form of the electrical signal which controls the light source.        a modulation of the intensity with zero mean with the addition of a bias current allowing the lighting function. The modulation used is of OFDM (acronym for “orthogonal frequency-division multiplexing”) type applied to LiFi. It makes it possible to control the illumination via the bias current, the addition of the OFDM signal with zero mean not modifying the level of the illumination. That means that the photodetector then transcribes the variations of the intensity of the light signal received. The photodetector is chosen such that there is no saturation of said detector linked to the intensity of the incident light, for example that from the sun or that from the LED.        
If the aim is to simultaneously produce the lighting and data transmission functions, it is necessary on the one hand to bias the LED module from a direct current (direct or DC component), and on the other hand to modulate the intensity of the LED module from a temporal analog signal with zero mean (alternating or AC component). This is why the so-called OFDM LiFi technology is suited to this type of dual function.
A conventional LED-based LiFi communication device comprises:                a data source (internet for example);        a specific electronic module making it possible to encode the data of the digital signal as analog signal;        an LED module;        in the case of the lighting/transmission function, a specific control means making it possible to add together the bias voltage (or current) and the analog signal containing the data to be transmitted;        a photodetector capable of detecting the modulated light signal and of transforming it into an electrical signal;        a signal processing module capable of using the electrical signal generated by the photodetector.        
The photovoltaic modules are photodetectors capable of transcribing a modulated light signal into a modulated electrical signal, that is to say corresponding to the variations of the light intensity and assumed representative of the form of the electrical signal which controls the light source. A communication system using photovoltaic modules as reception means is advantageous in that it makes it possible to dispense with the biasing (and therefore an addition of energy) of the photodetector and also makes it possible to consider delivering energy to the electronic components forming said reception means, for example to the signal processing module.
Generally, the photovoltaic modules are optimized to produce the maximum of energy by means of an impedance matching. The literature proposes a large number of solutions on the control algorithm performing a search for the maximum power point (commonly called MPPT, the acronym for “maximum power point tracking”) when the photovoltaic module and the load are connected through a solid-state converter. The I-V (intensity over voltage) characteristic of the photovoltaic module depends on the level of illumination, of the temperature and of the aging of said module. However, this I-V characteristic, when the light is modulated, is also a function of the frequency, of the type of modulation and of the associated modulation depth.
In order, at each instant, to extract the best modulated signal, it is necessary to introduce an impedance matching stage between the photovoltaic module and the signal processing module in order to couple the two elements as perfectly as possible; however, the known devices allowing an impedance matching are not suitable (because they are designed to extract the maximum of power), and do not therefore give satisfactory results in terms of faithful translation of the modulated light signal received by the photovoltaic module. When the load impedance of a photovoltaic module is not optimized, the analog electrical signal generated by the photovoltaic module is either too distorted for the information that it contains to be able to be processed, or too weak to be processed by the signal processing module.
Electronic means are known, notably through the document “High-speed visible light communication systems” by GROBE LILIANE ET AL, IEEE Communications Magazine, IEEE Service Center, Piscataway, US, Vol. 51, No. 12, 1 Dec. 2013 (2013 Dec. 1), pages 60-66, for amplifying the signal at the output of the photodetector. In the case where the electrical signal has been correctly restored at the output of the photodetector, the use of a trans-impedance amplifier (TIA) will make it possible to increase the level of the signal and consequently the signal-to-noise ratio (SNR) and therefore potentially the throughput. On the other hand, if the signal has not been correctly restored because, for example, of a poor impedance matching of the photodetector, the TIA, whose function is not to match the impedance in order to correct the distortion of the electrical signal, will only amplify the distorted signal, causing a deterioration of the SNR and therefore of the throughput. The TIA or the equivalent electronic amplification systems therefore do not aim to search for the maximum of SNR because they search only to maximize the level of the signal.